1. Field of Invention
The present invention relates to obtaining and processing seismic data.
2. Description of Prior Art
Noise reduction is of particular importance in the practice of seismic exploration. One of the most widely used noise reduction techniques involves averaging several seismograms that have been obtained by essentially repeating the seismic exploration sequence. A seismic source is initiated, a seismogram is recorded and the procedure is repeated a number of times. The recorded seismograms are presented to a summing device either on location or at a computer facility remote from the location of the seismic survey. By summing, the noise is averaged down in magnitude relative to the coherent signals that are sought as indicators of subsurface conditions. The signal-to-noise ratio is thereby improved.
The number of seismograms to average is limited by the cost of repeating the experiment, the repeatability of the source and the destructive characteristics of the source. For example, when dynamite is used as the source it is rare when more than one seismogram is obtained at each shot point. Averaging to overcome noise is replaced by costly procedures such as deepening shot holes and/or using larger charge sizes. When either the weight drop, hydraulically driven land impulse source, or explosive gas driven plate land impulse source is used as the source, perhaps as many as 100 individual seismograms may be obtained and summed at one shot point location. In these operations, time is the expense. Each seismogram requires about thirty seconds for the weight drop method and about ten seconds for the other two, so the time required for each shot point can be quite large, i.e. 15 to 50 minutes. The result obtained is a seismogram of less than six seconds duration which hopefully has an acceptable signal-to-noise ratio.
The swept frequency technique, which has to a large extent replaced weight dropping and the explosive gas driven plate land impulse source, combines the time-consuming averaging process with a computer-intensive correlation process that requires costly computing equipment to produce results that are sensitive to the choice of parameters for the frequency sweep function. In current swept frequency operations, about fifteen 20-second seismograms are taken and summed for each vibration point, requiring five to ten minutes at each vibration point. The 20-second summed seismogram is later correlated with the sweep function using expensive computing equipment at a seismic processing center to produce a representation of a recovered seismogram that is less than ten seconds duration. Furthermore, the representation of the recovered seismogram can be changed considerably by changing the sweep function that drives the vibrator source equipment and/or the sweep function used in the correlation process.